Thermodynamic Theory of Microemulsion Formation

When starting from the oil phase with dissolved surfactant and adding water. Solubilisation of the latter wiU take place and Solubilisation will increase with a reduction of temperature near the haze point. Between the Solubilisation and haze point curves, an isotropic region of W/O solubilised system exists. At any given temperature, any increase in water weight fraction above the Solubilisation limit will result in water separation (W/O solubilised-F water), while at any given surfactant concentration any decrease in temperature below the haze point will result in separation to water, oil, and surfactant. 

With nonionic surfactants, both types of microemulsions can be formed, depending on the conditions. With such systems, temperature is the most cracial factor as the solubility of surfactant in water or oil is temperature-dependent. Microemulsions prepared using nonionic surfactants will have a limited temperature range. 

The spontaneous formation of a microemulsion with a decrease in free energy can only be expected if the interfacial tension is so low that the remaining free energy 

The spontaneous formation of the microemulsion with decreasing free energy can only be expected if the interfadal tension is so low that the remaining free energy of the interface is overcompensated for by the entropy of dispersion of the droplets in the medium [7, 8]. This concept forms the basis of the thermodynamic theory proposed by Ruckenstein and Chi and Overbeek [7, 8]. 

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According to the thermodynamic theory of microemulsion formation, the total interfacial tension of the mixed film of surfactant and cosurfactant must approach zero. The total interfacial tension is given by the following equation. 

Several theories have been proposed to account for the thermodynamic stability of microemulsions. The most recent theories showed that the driving force for microemulsion formation is the ultralow interfacial tension (in the region of 10 4-10 2 mN m 1). This means that the energy required for formation of the interface (the large number of small droplets) A Ay is compensated by the entropy of dispersion —TAS, which means that the free energy of formation of microemulsions AG is zero or negative. 

The status of the systems commonly referred to as microemulsions among surface and colloid chemists is still somewhat uncertain. Various experimental approaches have been used in an attempt to ascertain the details of their structural and thermodynamic characteristics. As a result, new theories of the formation and stability of these interesting but quite complex systems are appearing. Although a great deal has been learned about microemulsions, there is much more to be learned about the requirements for their preparation and the relationships among the chemical structure of the oil phase, the composition of the aqueous phase, and the structures of the surfactant and the cosurfactant, where needed. 

Three key theories to explain microemulsion formation of have been proposed the mixed film, solubilization and thermodynamic theories. As described, these theories are not mutually exclusive, as elements from each can contribute to an understanding of microemulsion formation and stability. 

The formulation of microemulsions or micellar solutions, like that of conventional macroemulsions, is still an art. In spite of exact theories that have explained the formation of microemulsions and their thermodynamic stabihty, the science of microemulsion formulation has not advanced to a point where an accurate prediction can be made as to what might happen when the various components are mixed. The very much higher ratio of emulsifier to disperse phase which differentiates microemulsions from macroemulsions appears at a first sight that the appHcation of various techniques for formulation to be less critical. However, in the final stages of the formulation it can be realised immediately that the requirements are critical due to the greater number of parameters involved. 

This new theory of the non-equilibrium thermodynamics of multiphase polymer systems offers a better explanation of the conductivity breakthrough in polymer blends than the percolation theory, and the mesoscopic metal concept explains conductivity on the molecular level better than the exciton model based on semiconductors. It can also be used to explain other complex phenomena, such as the improvement in the impact strength of polymers due to dispersion of rubber particles, the increase in the viscosity of filled systems, or the formation of gels in colloids or microemulsions. It is thus possible to draw valuable conclusions and make forecasts for the industrial application of such systems. 

The thermodynamics of microemulsion discussed in the beginning of the chapter has accounted for the basic conditions required for the formation and stability of reverse micellar systems. The energetics of formation in terms of Gibbs free energy, enthalpy, and entropy need to be quantified with reference to the system composition and the droplet structures. For the formation of w/o systan, a simple method called dilution method can exfiact energetic information for many combinations along with the understanding of their structural features. The method has been amply studied and presented in literature [4,27-32]. We, herein, introduce and present the method with basic theory and examples. 

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